After you try your luck with the fortune teller in Belome Temple, you must battle your old foe Belome.
You might want to bring protection from sleep spells. Belome will sometimes eat one of your party members, then create a clone of that character who fights alongside him. The clones can be instantly defeated with Pure Water. The clones have various attacks depending on which character they are. The Princess clone can use Recover spells, the Mario clone can do jump attacks, the Mallow clone can attack with lighting spells, and so on. Even after you defeat Belome, you will have to defeat the clones if there are any remaining.
After the battle, jump onto the green switch to open the gate. Go through it. Talk to the mouse if you wish, then go down the pipe to reach Monstro Town.